1. Field of the Invention
The present invention relates to a compact, lightweight, and inexpensive portable system for metal detection and identification generally and, in particular, for landmine detection and classification.
2. Background
The World Health Organization estimates that 30,000 people are maimed or killed annually by landmines. Eight out of ten injured are civilians and seven out of ten are children. Between 100 million to 200 million unexploded mines lie buried worldwide and the United Nations (UN) estimates that five million additional mines are planted each year.
Landmines are low-priced, simple to construct and easy to plant. While de-mining, or clearing minefields, is expensive, time-consuming and d costs about $3 to make and moments to plant. But according to the UN, using present methods it would cost $33 billion to clear the world""s current minefields. The price of lost lives, personal injuries, and economic rehabilitation for landmine-infested areas is not included in that cost.
Landmines are made up of three basic parts: a container, an explosive charge, and a detonation device. However, de-mining is made even more difficult because mines are made in a wide variety of shapes and sizes with many different materials. Such variety makes it difficult to calibrate sensors to reliably detect mines. Today landmine containers are generally created from plastic, so they are not detectable by conventional metal detectors. Also, many mines are small enough to fit into a child""s hand. These small packages are dispersed over large areas, buried one to two inches underground, and can be very difficult to detect and neutralize.
Clearing landmines is slow and dangerous work. The casualty rate for de-mining personnel is near one casualty to 1000 mines cleared, but depending on the techniques used this rate can run as high as one casualty to 100 mines cleared. Crude mine clearing efforts frequently involve just an individual with a simple metal detector and a long stick. Although many more sophisticated approaches to mine clearing have been devised, they are generally very expensive. Heavily armored tanks that flail the ground with chains to intentionally detonate mines have been used on the battlefield since D-day. De-mining vehicles have also employed plows and giant rakes, and low-flying helicopters have dragged huge chain mats to explode and neutralize minefields.
To decrease the danger to de-mining personnel, many complex and expensive robotic devices have also been proposed. See, e.g., U.S. Pat. No. 6,026,135 to McFee et al., and U.S. Pat. No. 6,333,631 to Das et al. Recent high-technology proposals for detecting, classifying and assessing underground mines include the use of ground penetrating radar, ultrasonic sonar and passive infrared and microwave sensing. These technologies address the effectiveness of detecting mines, but do not address the portability requirements for individual soldier de-mining, nor the low-cost requirements for civilian de-mining applications.
Probably the greatest technical challenge for mine detectors is the ability to effectively discriminate mines and unexploded ordnance (UXO) from other underground debris. Current state-of-the-art electromagnetic induction (EMI) metal detectors can detect the small amount of metal in plastic-cased landmines at shallow depths under a wide range of environmental and soil conditions. However, small metal non-mine objects (i.e., clutter) commonly found in the environment are a major complication in mine detection because they represent false targets. It has been estimated that between 100 and 1000 false alarms occur from metal clutter in the environment for every real landmine detected. For time-efficient and cost-effective landmine clearing, the detected metal targets must be classified as to their threat potential: mine or clutter. In addition to a detection and classification sensor system, an effective, portable and inexpensive robotic platform is also needed to scan the sensor system over the minefield, thus removing de-mining personnel from eminent danger.
The present invention is a metal detector system that includes a chassis for supporting electromagnetic sensor components above a medium such as soil or water. A transmitter coil is attached to the chassis and induces an electromagnetic field in the medium beneath the chassis. The electromagnetic field creates eddy currents in metal objects in the medium. As the eddy currents decay, a first receiver coil is attached to one end of the chassis for receiving electromagnetic signals from the objects in the medium beneath the chassis. A second receiver coil attached to another end of the chassis also receives electromagnetic signals from the objects in the medium beneath the chassis. A propulsion system is attached to the chassis between the first and second receiver coils, or adjacent to the first and second receiver coils. The location of the propulsion system causes electromagnetic interference signals emanating from the propulsion system to be received at a nominally equal magnitude by each of the receiver coils. Finally, signal processing components add or subtract the outputs of the receiver coils, whereby interference signals emanating from the propulsion system and received by the receiver coils are nulled.